Insects: know your friends from your foes

Hannelene Badenhorst, a PhD candidate in entomology at the University of the Free State, explains the value of differentiating beneficial insects and other arthropods from harmful ones before resorting to blanket chemical control.

House/stable flies
House/stable flies (top: family Muscidae) and blowflies (bottom: family Calliphoridae) pollinate certain flowering plants.
Photo: Supplied by Hannelene Badenhorst

The estimated 2,5 million to 10 million species of insects on the planet have a profound impact, directly and indirectly, on all other creatures and on the environment.

This stands to reason; there are an estimated 10 quintillion (or 10 billion) individuals, and they represent 80% of all living organisms on earth.

They are also the most diverse group of organisms.

“Slightly more than one million insect species have actually been named, and only a small proportion of them have been properly studied,” says Hannelene Badenhorst, a PhD candidate in entomology at the University of the Free State.

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“They are responsible for pollinating about 85% of the world’s flowering plants.

“Non-insect arthropods, such as spiders, also have a significant environmental role. Some aid in predating on pests and by promoting the decomposition of organic matter, while others are pests themselves.” (Insects comprise 90% of all arthropod species.)

Although insects are the main consumers of plants, including crops, fewer than 1% of the known insect species are considered pests by humans, explains Badenhorst. Of these, only a few hundred species have proved to be constantly problematic.

All the other insects and arthropods are beneficial to humans and the environment, providing a diverse range of key ecosystem services between them.

One of these, she points out, is plant pollination. While bees make the largest contribution to pollination and rightfully deserve to be protected, it is also critical to maintain the diversity of other insect pollinators, as many plant species have co-evolved with them and can only be pollinated by specific groups of pollinators.

Fighting fire with fire
A second ecosystem service provided by insects and non-insect arthropods is biological control of pests and diseases. In agriculture it is known that predatory arthropods, such as ladybirds, feed on damage-causing arthropods, such as aphids, making a considerable contribution to regulating their populations and reducing the possibility of a pest outbreak.

Less well known is that some arthropods, such as ground beetles, feed on weed seeds, thereby reducing weed populations in croplands. Other arthropods feed on fungi, including those that can attack crops.

“One of the most important and well known, yet widely misunderstood and underappreciated, groups of predatory arthropods is spiders,” explains Badenhorst.

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“It has been estimated that the global spider community collectively kills 400 million to 800 million tons of prey annually.

“This makes spiders invaluable for managing populations of arthropods, especially those considered pests in agricultural ecosystems.”

Badenhorst says many beneficial arthropod species spend part or all of their life cycle in the soil. It is therefore important that farmers protect both above-ground and below-ground populations by minimising soil disturbance that could negatively affect the diversity and numbers of beneficial below-ground arthropods.

The shredding and decomposition of organic materials is a third key ecosystem service provided by insects and non-insect arthropods. Millipedes, sowbugs, termites, mites, springtails and cockroaches are some of the arthropods with strong mouthparts capable of fragmenting plant and animal residue lying on the soil surface.

The fragmented organic material becomes available to smaller arthropods, bacteria and fungi, which then feed on it and decompose it down to a molecular level.

This decomposed organic matter filters down into the soil profile, helping to improve soil structure and health, and provide nutrients to plants, including crops, growing in the soil.

Badenhorst points out that some arthropod species provide a fourth key ecosystem service: aerating the soil and improving its capacity to capture and retain water. They include ants, beetles, termites, millipedes and even bees. Like earthworms, who are renowned and treasured for having similar abilities, these arthropods are also “engineers of the soil”.

Underground highways
“The actions of the many arthropods that spend part or all of their life cycle in the soil result in an extensive network of burrows and tunnels throughout the soil profile, both vertically and horizontally,” explains Badenhorst.

“These subsoil spaces enable oxygen from the atmosphere to reach all life within the soil, and rain and irrigation water to filter through the soil profile. The actions of these soil-dwelling arthropods also give the soil a spongy structure.

“In the absence of these organisms, soils can become almost as hard as a brick, with the result that water runs off the surface, no oxygen can penetrate, and almost nothing can grow below ground.”

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The fifth key ecosystem service provided by arthropods is nutrient cycling and soil mixing. Many of these creatures feed on fungi and bacteria, as well as the fine roots. The action of feeding ultimately releases nutrients from these foods back into the soil environment for other life to utilise.

Earthworms and larger arthropods, such as dung beetles, feed on organic material on or near the soil surface, and either draw part of, or excrete nutrients from, these foods down into the soil profile.

Functional diversity within the dung beetle group comprises endocoprids, which burrow and nest within, or just below, fresh dung piles; telecoprids, which remove a ball of dung from a fresh pile and bury the ball in a tunnel; and paracoprids, which dig deep tunnels below the dung pile, move dung into these tunnels, and then lay their eggs inside the dung balls.

“All plant-feeding insects exhibit some degree of selectivity for which plants they feed on,” explains Badenhorst. “This includes selecting the host plant to feed on, as well as finding an appropriate individual plant of high enough quality for feeding, survival and the development of their nymphs or larvae.”

To identify a potential host plant over area and distance, insects use their sense of smell or vision, or a combination of both.

Different plants emit odours specific to themselves, which insects are able to distinguish between. On the other hand, colours and shapes can be similar within and between various plant species.

However, the combination of odour, colour and shape can be a distinctive identifier for insects searching for a potentially suitable host plant. Once an insect has reached such a plant, it will make a final close-up olfactory, visual, taste and tactile evaluation before choosing to settle or to move on to find a better plant.

Integrated pest management
Understanding how and why insects select host plants is a valuable tool for farmers and their pest management consultants in developing an ecologically sensitive and targeted integrated pest management (IPM) plan.

Badenhorst says a crucial element of such an IPM is to grow multiple plant species among the primary crop.

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“Some pest species avoid lands with multiple plant species due to the high energy costs of seeking out and identifying the preferred host plant amongst many other. These pests tend to make for a monoculture cropland.”

A further important aspect of IPM is to limit mechanical soil disturbance and pesticide application in a cropland as much as possible.

According to Badenhorst, minimum tillage and no-pesticide practices significantly improve the diversity and survival of predator insects and other organisms, such as soil microbes, that are beneficial to the cropping enterprise.

“Agricultural lands are connected to everything around them, so they should be managed as part of a larger ecosystem.

“For healthy ecosystems and for maximising ecosystem services, it’s important for farmers to increase habitat diversity. This provides shelter and food resources for all beneficial arthropods. Farmers should manage their operations as part of an agroecosystem,” says Badenhorst.

Email Hannelene Badenhorst at [email protected], or visit ufs.ac.za.

This article is based on a presentation at the 2019 No-Till Conference held at ATKV Drakensville, KwaZulu-Natal from 3 to 5 September 2019.